Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/04/30/determining-when-the-datefield-control-is-opened-or-closed-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.events.DropdownEvent;

            private function dateField_openClose(evt:DropdownEvent):void {
                arrColl.addItem(evt);
            }
        ]]>
    </mx:Script>

    <mx:ArrayCollection id="arrColl" />

    <mx:ApplicationControlBar dock="true">
        <mx:DateField id="dateField"
                open="dateField_openClose(event);"
                close="dateField_openClose(event);" />
    </mx:ApplicationControlBar>

    <mx:DataGrid id="dataGrid"
            dataProvider="{arrColl}"
            itemRenderer="mx.controls.Label"
            width="100%"
            height="100%" />

</mx:Application>

View source is enabled in the following example.

And here is the “same” example, but in ActionScript instead of MXML:

comps/MyDateField.as

/** http://blog.flexexamples.com/2008/04/30/determining-when-the-datefield-control-is-opened-or-closed-in-flex/ */
package comps {
    import mx.collections.ArrayCollection;
    import mx.containers.VBox;
    import mx.controls.DataGrid;
    import mx.controls.DateField;
    import mx.controls.Label;
    import mx.core.Application;
    import mx.core.ClassFactory;
    import mx.events.DropdownEvent;

    public class MyDateField extends VBox {

        [Bindable]
        private var arrColl:ArrayCollection;

        private var dateField:DateField;
        private var dataGrid:DataGrid;

        public function MyDateField() {
            super();
            init();
        }

        private function init():void {
            arrColl = new ArrayCollection();

            dateField = new DateField();
            dateField.addEventListener(DropdownEvent.OPEN, dateField_openClose);
            dateField.addEventListener(DropdownEvent.CLOSE, dateField_openClose);

            dataGrid = new DataGrid();
            dataGrid.dataProvider = arrColl;
            dataGrid.itemRenderer = new ClassFactory(mx.controls.Label);
            dataGrid.percentWidth = 100;
            dataGrid.percentHeight = 100;

            Application.application.appControlBar.addChild(dateField);

            addChild(dataGrid)
        }

        private function dateField_openClose(evt:DropdownEvent):void {
            arrColl.addItem(evt);
        }
    }
}

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/04/30/determining-when-the-datefield-control-is-opened-or-closed-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        xmlns:comps="comps.*"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:ApplicationControlBar id="appControlBar" dock="true" />

    <comps:MyDateField width="100%" height="100%" />

</mx:Application>

View source is enabled in the following example.